The small to medium size photocopying machine usually has what is called a "straight-through" paper path. Thus, the plain paper onto which the orginal is to be copied is fed in a straight line through the copier from one end (where it is automatically loaded in from a hopper or cassette) to the other (where it is ejected into a collecting tray). As it progresses through the machine the paper is first contacted with a powder-bearing charged photosensitive surface and then delivered to and though a powder fusing section. The difficulty when copying books, is that the book must be opened wide enough to allow it to be placed face down flat on the glass platten, and this usually results in damage to the spine of the book, as the book is flattened out.
Various attempts have been made to deal with this, but none are particularly successful. In one type of copier, for example, the platten is positioned so as to extend right up to the very left-hand edge of the machine. A book may therefore be copied by placing it face down on the platten with only one half (one page) on the platten itself, the other half hanging down the vertical side of the machine, thereby avoiding the need to "flatten" the book. Extending the platten right up to the left-hand edge does create problems since it it difficult to arrange the optics of the imaging system such that the extreme left edge area of the original is properly seen and imaged onto the photosensitive surface, especially where reduction or enlargement is required. Furthermore the weight of the hanging portion of the book can cause the book to fall if the operator lets go without thinking.
Moreover, a trivial but irritating problem arising from the very nature of a straight-through paper system is that necessarily the hanging-down portion of a book may easily block the exit path of the formed copy.